1. Field of the Invention
The present invention relates to a novel flash butt welding method and a welding apparatus and more particularly it relates to a flash butt welding method and a welding apparatus having the specific means for applying voltage.
2. Description of the Prior Art
A single-phase AC power source having the commercial frequency (sine waveform) has been mainly used as the power source for the flash butt welding operation. When a local short-circuit of workpieces is caused near the zero of the voltage waveform of FIG. 1 (the shaded lines of FIG. 1) in the case of said power source, the voltage is low to limit the current passing through the short-circuited part whereby the short-circuit continues until the short-circuited part is scattered by raising the voltage without changing to the flash. During this period, the distance between the workpieces is shortened by moving the movable electrode whereby the effective area for current passing between the edge surfaces of the workpieces is increased, but the current per mm.sup.2 is not increased. Accordingly, the edge surfaces of the workpieces are not heated as desired, and the flash is not formed. At last, a butting (the condition causing the short-circuit between the workpieces without the flash) may result.
Accordingly, it has been necessary to apply a high voltage (hereinafter referring to as the flash voltage) so as to heat the edge surfaces of the workpieces by increasing the current per mm.sup.2.
When the flash voltage is high, the input power source capacity must be increased to cause a rough flash whereby the quality of the welded part is disadvantageously effected.
It has been proposed to decrease the flash voltage by using the DC waveform of the flash voltage in order to prevent the increase of the flash voltage. However, when the sectional area of the workpiece is large and a large output capacity of the power source is required, the effect of the inductance at the part of the rectifier can not be negligible to the impedance of the main circuit whereby the rising of the flash current is remarkably limited to deteriorate the function of the flash butt welding apparatus.
It has been also proposed to control the flash voltage by the phase control with a thyristor etc. when a sine waveform AC power source is used. FIG. 2 shows the voltage waveform in said case wherein the period applying no voltage (A-B) is prolonged to cause further disadvantage.
It has been disclosed the fact that a quality of a welded part is inferior in the flash welding operation by the phase controlled voltage in "Welding Journal Vol. 50, No. 5 1971 Pages 213 s to 221 s", "Effect of phase control during flashing on flash weld defects".
The conventional flash butt welding apparatus has the following disadvantages.
(1) A sine waveform AC power source is mainly used. The flash voltage should be increased for maintaining the flash with the sine waveform AC power source. The input power source capacity should be increased to cause a rough flash.
(2) The rise of the flash current is limited by the effect of the inductance of the rectifier in the case of a DC power source especially one having large capacity, whereby the flash is not preferably formed.
(3) The phenomenon of (1) is further remarkable by using the sine waveform AC power source under the phase control.